This invention relates to a process and apparatus for separating liquid, especially for separating sulfur from a sulfur-lye suspension, in which the sulfur-lye suspension is fed into a separation tank. The temperature inside the separation tank is maintained above the melting temperature of sulfur so that liquid sulfur lye can be separated by gravity and then withdrawn from the separation tank. (By "lye" it is intended to define any alkaline solution, particularly a sodium carbonate solution.)
By the use of the technique of oxidative scrubbing of hydrogen sulfide from a flue gas, hydrogen sulfide is removed and immediately oxidized to elementary sulfur. An aqueous alkaline solution, containing essentially sodium carbonate, vanadium salts, and organically bonded nitrogen, is usually used as a regenerable scrubbing liquid (e.g., DE-OS 32 39 602).
Several reactions generally occur in the scrubbing step. The absorbed hydrogen sulfide dissociates in the alkaline solution into HS- ions forming NaHS. This sodium hydrogen sulfide is completely oxidized by vanadate V ion into elementary sulfur, which is suspended in the scrubbing liquid in the form of fine solid particles. Simultaneously, the vanadium is reduced from oxidatively active valence V to oxidatively inactive IV.
The resultant scrubbing liquid made inactive through H.sub.2 S oxidation is aerated with air in an oxidizing vessel to reconvert the vanadium to valence V. In aerating the scrubbing liquid, the suspended elementary sulfur becomes concentrated at the liquid surface as sulfur foam and is drained into the collecting tank through an overflow weir. The oxidized lye is passed into a surge tank to be later treated for sulfur removal so that sulfur-free solution can be recycled to the flue gas scrubbing step.
The initial separation of sulfur from the scrubbing liquid is conducted with centrifuges or decanters, producing a sulfur cake with a residual moisture of 60 to 70% by weight. For complete sulfur-lye separation, the sulfur is melted and decanted from the scrubbing liquid. During this last step, the undesirable by-product sodium thiosulfate Na.sub.2 S.sub.2 O.sub.3 is formed. Its rate of formation should be kept as low as possible.